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A detailed and systematic microstructural characterization has been carried out on a Ti-47Al-2Cr-2Nb (at. pct) intermetallic alloy processed by powder metallurgy (PM). Heat-treatment parameters such as isothermal temperature, holding time, and cooling rate were varied in order to produce a series of near-c, duplex, and fully lamellar microstructures. These were then quantitatively analyzed in terms of grain size, surface fraction, lamellar spacing, second-phase spatial distribution, and serrated grain boundary morphology. Owing to these extensive quantitative image analyses, several unusual microstructural features occurring in this wellknown TiAl-based alloy were identified and assessed. First, a dissolution of the smallest c grains was emphasized in subtransus conditions as the isothermal temperature or holding time was increased. Second, the competition that occurs between the a Þ a + c transformation and the direct-ordering a Þ a 2 reaction upon cooling from above the a-transus temperature is mainly governed by the reduction in chemical free energy. Third, new grains were found to nucleate upon cooling, which is presumably induced by a minimization of interfacial energy at prior a grain boundaries. Finally, new c grains were formed as a result of the coarsening of primary c lamellae under furnace-cooled (FC) conditions.

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